Reaction Wheel Assemblies are used on LEO and GEO satellites to control errors in Pitch, Roll and Yaw axises. The assembly consists of the electronics to monitor and control the wheel, a motor and the wheel its self. A Reaction wheel works by applying current to a motor that spins a wheel with a given mass and is based on the common principal that every action has an equal and opposite reaction. By applying current to increase or decrease the wheel speed control torques can be generated in the given axis. As the wheel is spun faster it gains stored momentum in the axis and therefore in systems that employ reaction wheels, periodic momentum management is required to prevent the wheel from exceeding its recommended speed range. Momentum unload procedures typically use opposing thruster firings to drive the wheel speed down or up and reduce the stored momentum.

In a zero momentum biased control system when the unload procedure is activated, it drives each axises momentum to zero momentum. In advanced control systems this function is enabled during the standard station keeping maneuvers and can conserve fuel.

When a fourth wheel is included for redundancy, the wheel’s orientation is set such that each wheel has an influence on 2 axis and the ACS or ADACS system uses the alignment of each wheel and axis to calculate the required torque and distribute them over the wheels to control errors in an axis. In this configuration a minimum of three wheels are required to operate the satellite and the fourth wheel can be turned on when needed to replace a failed or failing wheel. The system can also distribute the torques over all four wheels if desired, providing it has been designed to do so.